Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of communicating the signals over long distances with very low loss.
Optical networks often employ modulation. In general, modulation is the process of varying one or more properties (e.g., amplitude, phase, and/or frequency) of a particular periodic waveform (usually a high-frequency waveform), called the carrier signal, with respect to a modulating signal. Thus, modulation may allow optical networks to convey a message signal (for example a digital bit stream or an analog audio signal), inside another signal that can be physically transmitted (e.g., an optical carrier signal propagating through an optical fiber).
A type of modulation often used in optical networks is phase modulation. Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave. A commonly-used phase modulator in optical networks is a Mach-Zehnder Modulator (MZM). In an MZM, a input beam of light may be split evenly between two matched arms of optical fiber, and then recombined into an output beam of light. Each arm may be proximate to a corresponding pair of electrodes, such that when a voltage is applied to an arm's corresponding pair of electrodes, the refractive index of the optical fiber of the matching arms varies, thus varying the speed light through the waveguide. Based on the voltage applied to each pair of electrodes, the output beam of light may be a phase or amplitude modulated version of the input beam (e.g., a voltage applied to both pairs of electrodes may induce a half-wave phase delay on the beam). Based on their phase shifting characteristics, MZMs may be used in a variety of phase modulation schemes, include binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), dual-polarization QPSK, and other formats. As phase modulation schemes increase in complexity, the number of MZMs required increases. Due to cost and expense associated with MZMs, a less expensive alternative may be desirable.